diff --git a/Sources/Overlays/_Testing_CoreGraphics/Attachments/AttachableAsCGImage.swift b/Sources/Overlays/_Testing_CoreGraphics/Attachments/AttachableAsCGImage.swift
index 4bb060377..0fbcac010 100644
--- a/Sources/Overlays/_Testing_CoreGraphics/Attachments/AttachableAsCGImage.swift
+++ b/Sources/Overlays/_Testing_CoreGraphics/Attachments/AttachableAsCGImage.swift
@@ -12,6 +12,9 @@
 package import CoreGraphics
 package import ImageIO
 private import UniformTypeIdentifiers
+#if canImport(UniformTypeIdentifiers_Private)
+@_spi(Private) private import UniformTypeIdentifiers
+#endif
 
 /// A protocol describing images that can be converted to instances of
 /// [`Attachment`](https://developer.apple.com/documentation/testing/attachment)
@@ -47,6 +50,20 @@ package protocol AttachableAsCGImage: AttachableAsImage {
   var attachmentScaleFactor: CGFloat { get }
 }
 
+/// All type identifiers supported by Image I/O.
+@available(_uttypesAPI, *)
+private let _supportedTypeIdentifiers = Set(CGImageDestinationCopyTypeIdentifiers() as? [String] ?? [])
+
+/// All content types supported by Image I/O.
+@available(_uttypesAPI, *)
+private let _supportedContentTypes = {
+#if canImport(UniformTypeIdentifiers_Private)
+  UTType._types(identifiers: _supportedTypeIdentifiers).values
+#else
+  _supportedTypeIdentifiers.compactMap(UTType.init(_:))
+#endif
+}()
+
 @available(_uttypesAPI, *)
 extension AttachableAsCGImage {
   package var attachmentOrientation: CGImagePropertyOrientation {
@@ -63,8 +80,21 @@ extension AttachableAsCGImage {
     // Convert the image to a CGImage.
     let attachableCGImage = try attachableCGImage
 
+    // Determine the base content type to use. We do a naïve case-sensitive
+    // string comparison on the identifier first as it's faster than querying
+    // the corresponding UTType instances (because it doesn't need to touch the
+    // Launch Services database). The common cases where the developer passes
+    // no image format or passes .png/.jpeg are covered by the fast path.
+    var contentType = imageFormat.contentType
+    if !_supportedTypeIdentifiers.contains(contentType.identifier) {
+      guard let baseType = _supportedContentTypes.first(where: contentType.conforms(to:)) else {
+        throw ImageAttachmentError.unsupportedImageFormat(contentType.identifier)
+      }
+      contentType = baseType
+    }
+
     // Create the image destination.
-    guard let dest = CGImageDestinationCreateWithData(data as CFMutableData, imageFormat.contentType.identifier as CFString, 1, nil) else {
+    guard let dest = CGImageDestinationCreateWithData(data as CFMutableData, contentType.identifier as CFString, 1, nil) else {
       throw ImageAttachmentError.couldNotCreateImageDestination
     }
 
diff --git a/Sources/Testing/Attachments/Images/ImageAttachmentError.swift b/Sources/Testing/Attachments/Images/ImageAttachmentError.swift
index e172abd8c..b1ad5a347 100644
--- a/Sources/Testing/Attachments/Images/ImageAttachmentError.swift
+++ b/Sources/Testing/Attachments/Images/ImageAttachmentError.swift
@@ -25,6 +25,9 @@ package enum ImageAttachmentError: Error {
 
   /// The image could not be converted.
   case couldNotConvertImage
+
+  /// The specified content type is not supported by Image I/O.
+  case unsupportedImageFormat(_ typeIdentifier: String)
 #elseif os(Windows)
   /// A call to `QueryInterface()` failed.
   case queryInterfaceFailed(Any.Type, CLong)
@@ -57,6 +60,8 @@ extension ImageAttachmentError: CustomStringConvertible {
       "Could not create the Core Graphics image destination to encode this image."
     case .couldNotConvertImage:
       "Could not convert the image to the specified format."
+    case let .unsupportedImageFormat(typeIdentifier):
+      "Could not convert the image to the format '\(typeIdentifier)' because the system does not support it."
     }
 #elseif os(Windows)
     switch self {
diff --git a/Tests/TestingTests/AttachmentTests.swift b/Tests/TestingTests/AttachmentTests.swift
index b457fc271..623ead084 100644
--- a/Tests/TestingTests/AttachmentTests.swift
+++ b/Tests/TestingTests/AttachmentTests.swift
@@ -597,6 +597,33 @@ extension AttachmentTests {
       }
     }
 
+    @available(_uttypesAPI, *)
+    @Test func attachCGImageWithCustomUTType() throws {
+      let contentType = try #require(UTType(tag: "derived-from-jpeg", tagClass: .filenameExtension, conformingTo: .jpeg))
+      let format = AttachableImageFormat(contentType: contentType)
+      let image = try Self.cgImage.get()
+      let attachment = Attachment(image, named: "diamond", as: format)
+      #expect(attachment.attachableValue === image)
+      try attachment.attachableValue.withUnsafeBytes(for: attachment) { buffer in
+        #expect(buffer.count > 32)
+      }
+      if let ext = format.contentType.preferredFilenameExtension {
+        #expect(attachment.preferredName == ("diamond" as NSString).appendingPathExtension(ext))
+      }
+    }
+
+    @available(_uttypesAPI, *)
+    @Test func attachCGImageWithUnsupportedImageType() throws {
+      let contentType = try #require(UTType(tag: "unsupported-image-format", tagClass: .filenameExtension, conformingTo: .image))
+      let format = AttachableImageFormat(contentType: contentType)
+      let image = try Self.cgImage.get()
+      let attachment = Attachment(image, named: "diamond", as: format)
+      #expect(attachment.attachableValue === image)
+      #expect(throws: ImageAttachmentError.self) {
+        try attachment.attachableValue.withUnsafeBytes(for: attachment) { _ in }
+      }
+    }
+
 #if !SWT_NO_EXIT_TESTS
     @available(_uttypesAPI, *)
     @Test func cannotAttachCGImageWithNonImageType() async {